Self-compensating optical system



l. c. GARDNER 2,511,840

SELF-COMPENSATING OPTICAL'SYSTEM FOR TELEMETERS June 20,: 1950 2Sheets-Sheet 1 Filed March 30, 1942 7 A 7 VMWWY Fri or Art.

amend/M Irvine \l Gardner 'l. c. GARDNER 2,511,840

SELF-COMPENSATING OPTICAL SYSTEA FOR TELEMETERS.

I 2 Sheets-Sheet 2 E- Gardner 'J une 20, 1950 Filed Marc h so, 1942Patented June 20, 1950 SELF-COMPENSATING OPTICAL SYSTEM FOR TELEMETERSIrvine C. Gardner, Chevy Chase, Md., assignor to the United States ofAmerica as represented by the Secretary of War Application March 30,1942, Serial No. 436,832

9 Claims. (Cl. 88-23) (Granted under the act of March 3, 1883, asamended April 30, 1928; 370 0. G. 757) The invention described hereinmay be manufactured and used by or for the Government for governmentalpurposes, without the payment to me of any royalty thereon.

This invention relates to improvements in optical telemeters and is moreparticularly adapted for use in rangefinders or heightfinders of theself-contained base type.

It is a primary object of the invention to provide a telemeter of thecharacter referred to which will be far less sensitive to changeseffected by external conditions than present day telemeters, whereby theerrors introduced by such changes may be greatly minimized or completelyobviated.

A further object of the invention is to provide a telemeter that willrequire only infrequent adjustment if any adjustment is required at all.

Still a further object of the present invention is to provide atelemeter having main optical systems possessing the same degree ofstability as that inherent in the present day internal adjustor.

Still a further object of the invention is to provide a telemeter ofsuch inherent stability that it may be focused for objects at differentdistances without loss of adjustment.

Through a comprehensive study of telemeters with a view of determiningthe errors therein and their sources I have found that the greatestinstability exists in the main optical systems. The measuring wedges andscales found in the modern telemeter are of good design and if properlyconstructed do not contribute to the variable performance of theinstrument. Similarly the elements of the optical system lying betweenthe two reticles and the eyes of the observer, if of workmanlikequality, will not affect the range reading. Further, translations orrotations of the end penta-reflectors within the limits corresponding togood mechanical construction do not introduce errors. The chief causesfor the inconsistent performance of the telemeter are the following:

1. Temperature gradients in the air within the body of the telemeter.

2. Bending of the optical tube. This may be produced by mechanicalstrains or by temperature gradients.

3. Variation in the deviations produced by the two penta-refiectors.These variations arise from non-uniform expansion resulting fromtemperature gradients.

4. Parallax between the target and the reticle as viewed through thetelemeter. This parallax is present because the telemeter of presentdesign does not permit the advantageous application of a focusingadjustment for varying the distance between each objective and itscorresponding reticle as is required if parallax is to be eliminated fortargets at different distances.

As to the first cause, temperature gradients produce gradients in theindex of refraction of the air and this deviates the rays travellingalong the axis of the telemeter body. The errors arising from this causeas well as a solution to the problem are disclosed in U. S. Patent1,968,267 to R. Straubel, dated July 31, 1934.

A solution of the problem created by the third cause is disclosed in mycopending application, Serial No. 432,412, filed February 26, 1942, nowPatent No. 2,400,111 dated May 14, 1946, and the instant disclosurereveals a solution of the problem caused by the second and fourth causesabove enumerated.

As is well known by those skilled in the art telemeters of theself-contained base type are provided with an internal adjustor which isan auxiliary optical system designed to produce an artificial targetoptically, at an infinite distance,

and adjustment consists in setting the instrument to bring the readingmade upon this target into agreement with the infinite graduation on therange scale. In order that the adjustor may satisfactorily serve itspurpose the optical system thereof is so designed and constructed thatits performance may be expected to be independent of variation resultingfrom temperature changes and mechanical strains which affect the mainoptical system. This freedom from variation should be within the limitsof precision of the telemeter. For a telemeter with a magnification of24 an angular'variation of a half-second is significant. Experience hasshown that prior art telemeters are so sensitive to slight deformationsresulting from temperature changes or mechanical strains that they mustbe frequently adjusted if serviceable readings of range or height are tobe secured. When the ambient temperature is varied there is aconsiderable period of time during which the telemeter is not in asteady state and, therefore, for perfect operation this process ofadjustment should be a continuous one. Where the magnification isincreased as it is proposed to do, in the case of heightfinders to 48,the difficulty of maintainin the telemeter in adjustment is greatlyaccentuated because the significant variation then becomes onequartersecond.

At present this instability of the prior arttelemeters is one of theirmost serious faults and it is a primary object of my invention toeliminate the instability of the main optical system thereof byproviding a system that is free from variation due to flexures arisingfrom temperature changes or mechanical strain to such extent that itwill be comparable with the internal adjustor above referred to in thisrespect.

The specific nature of the invention as well as other objects andadvantages thereof will clearly appear from a description of a preferredembodiment as shown in the accompanying drawings in which:

Fig. l is a diagrammatic sectional view of a conventional optical tubefor mounting the main optical systems as employed in prior artselfcontained base telemeters,

Fig. 2 is a view showing the efifect of a transverse temperaturegradient upon an' optical tube of the type shown in Fig. 1,

Fig. 3 is a diagrammatic sectional view of an optical tube systemarranged inaccordance with the present invention,

Fig. 4 is a view inelevation showing one of the .end plates used in theoptical tube system of Fig. 3,

Fig. 5 is a diagrammatic sectional view similar -to Fig. 3 showing theaddition to the optical tube system of the latter view of two prismswhich direct light after passage through the reticles to the erectorsand finally the eyepieces locatedcentrally of the telemeter, 1

Fig. 6 shows the structure of Fig. 5 as viewed in the plane of the paperfrom the side adjacent optical tube II1 with the latter :tube shown insection,

Fig. '7 is a diagrammatic view showing an optical system similar to thatof Figs. 3, 5, and 6 provided with a focusing adjustment.

Referring now to the drawings and more particularly to Fig. 1 there isshown an optical tube I of the type universally employed in telemetersof a conventional design before the present invention. Adjacent each endof the tube II) there is mounted an objective lens H1 or II2 of the mainoptical systems and in the focal planes of each of the objectives I I1or 'I I2 are mounted reticles I21 and I22, respectively. These reticlesbear the patterns which fuse to produce the wandering mark, and asunderstood .by those skilled in this art each objective forms an imageof the target upon its respective reticle. The two or main opticalsystems generally designated I31 and I32 are comprised of the objectiveI I1--reticle I21 and objective IIz-reticle I22, respectively,'and aremounted in such manner as to have their axes I41 and I42 nominally lyingin the same straight line. Any variation in the angle between the axesI41 and I42 must be compensated for by an adjustment of the telemeter ifthe correct range is to be determined by the telemeter. In telemeters ofthe type above referred to the optical tube is comparatively slender forits length and consequently is subject to flexing under the slightestmechanical strain. As a matter of fact the optical tube is so sensitiveto flexure that it must be carefully supported in a heightfinder toavoid significant fiexures introduced by its own weight when theinstrument is trained on targets at different angular heights. As aresult of my research I have found that this type of optical tube isparticularly responsive to a temperature gradient. For a thirteen-foottelemeter the optical tube is two meters long and as shown in Fig. 2 agradient I transverse to the axis I6 of the tube (which contains theaxes I41, I42) amounting to 0.01 C. per millimeter (which correspondsapproximately to a temperature difierence of 1 degree between the topand bottom of the tube) will cause the tube to bend through an angle aof approximately 40 seconds measured as the change in angle betweentangents to the axis of the tube at its extreme ends.

In Fig. 3 I have shown an optical tube system in accordance with mypresent invention which will not be subject to the efiects of atransverse temperature gradient as is the case of a tube of the formshown in Fig. 1. The tube system comprises two optical tubes I11 and H2arranged side by side with their axes I81 and I82 disposed inparallelism. Each tube mounts one of the optical systems 2I1 or 2I2comprising the main optical systems of the telemeter. The optical system2I1 comprises the objective lens I91 and reticle 201 while the opticalsystem 2I2 comprises the objective lens I92 and reticle 282. At each endof the tubes the adjacent objective lens and reticle cells may bemounted in suitable plates 22 formed of a material having an extremelylow coefficient of expansion. I have found fused silica verysatisfactory for this purpose as its coefiicient of expansion is onlyone twenty-fifth that of steel. The plates 22, as clearly shown in Fig.3, also serve to secure the tubes I I1 and I I2 in their proper relativepositions above described. By arranging the two optical systems 2I1, 2I2in this manner a tube system is provided wherein its length as comparedwith the length of the single optical tube universally employedheretofore has been substantially reduced with a resulting greatreduction in the likelihood of flexure of the system due to mechanicalstrain. Furthermore, such mechanical flexure as may arise will not alterthe parallelism of the axes I81, I82 of the two optical systems 2 I1, 2I2 since the spacings of the objective I 91, recticle 202, and objectiveI 92, reticle 281 remain unchanged independent of such flexure.Moreover, by arranging the tubes I11, I12 in the manner shown in Fig. 3,whether or not the fused silica supports 22 are used, a transversetemperature gradient will not alter the parallelism of the axes I81, I82of the two optical systems 2I1, 2I2 for the same reason that the errorsnormally arising from mechanical flexure have been obviated. Alongitudinal temperature gradient will afiect the parallelism of theaxes I 81, I 82 because the temperature at the opposed ends of the tubesystem will be different with consequent diifering amounts of expansion.However, if the fused silica end plates 22 are employed and alongitudinal temperature gradient of 0.01 C. per millimeter is assumed,the total angular change will amount to approximately 0.1 second, whichis a great improvement over the performance of the type of mounting forthe main optical systems shown in Fig. 1. I

In Figs. 5 and 6 the optical tube system of Fig. 3 is shown with prisms231 and 232 added to direct the light after passage through the reticles282 and 201. respectively, to erector prisms and finally to theeyepieces located in the usual position near the center of thetelemeter. The erectors and eyepieces form no part of the presentinvention and have, therefore, been omitted for the sake of clarity. Ashereinbefore referred to, variations in the performance of the prisms231 and 232 will not affect the accuracy of the range reading of thetelemeter because they cannot change the relative positions of theimages formed on the reticles since this latter relation is dependentupon the relative positions of the objectives and reticles alone.

Telemeters of the type having optical tube systems similar to the oneshOWn diagrammatt cally in Fig. l are not provided with focusingmovement for the purpose of varying the distance between objective l hand reticle I21 or objective H2 and reticle I22, because any mechanicalmovement effecting a change in the spacing of these elements would as aconse quence of the normal unavoidable inaccuracies ,of constructionthereof introduce components of movement transverse to the axis of theoptical tube system and thereby materially disturb the adjustment of thetelemeter in a manner similar to that arising from subjecting thetelemeter to mechanical flexures or flexures attributable to thepresence of temperature gradients as above described. In an optical tubesystern arranged in accordance with my invention, however, I am enabledto introduce focusing movement of the objectives and reticles withoutaccompanying disturbance of adjustment of the telemeter and to this endI have shown in Fig. 7 an optical tube system similar to that shown inFig. 3 but provided with a suitable focusing movement.

In the structure of Fig. 7 the optical tubes Ila, H4 of the system areeach comprised of two sections (llel'iv) and (Ila-Ila), respectively,arranged in telescopic engagement with each other for relativelongitudinal movement. The focusing movement comprises a rack 23suitably attached to one of the plates 22 with its teeth disposed inmeshing relation with a pinion 2 suplportably journaled, in anydesirable manner on the telemeter housing whereby rotation of the pinionwill translate the rack and simultaneously move the optical tubesections relative to each other to change the spacing of objective I92and reticle 292 and of objective I91 and reticle 281. As will beobserved the focusing movement just described is so constructed thatobjective I91 and reticle 202 are translated as a unit with respect toobjective I92 and reticle 231 in a direction parallel to the commondirection of the parallel axes I81 and 182. It follows that theobjective I91 and reticle 202 and of objective I92 and reticle 21hremain fixed as in the case of the structure of Fig. 3 and that anyresulting transverse components of movement resulting from theunavoidable inaccuracies of the focusing movement will not alter theparallelism of axes I81 and I82.

In all forms of the invention the main optical systems of the telemeterare so mounted as to be self-compensating for distortions introduced bymechanical strain or temperature changes within the limits of precisionof such instruments.

Having now described what is at present considered to be a preferredembodiment of the invention, I claim:

1. In a telemeter of the self-contained base type, an optical tubesystem comprising a pair of tubular members arranged side by side onparallel axes, an objective mounted on the end portion of one of saidtubular members, a second objective mounted on the diagonally opposedend portion of the other one of said tubular members, a. reticle mountedon the end portion of each tubular member remote from the objective insaid tubular member, said objectives being of identical focal length andthe length of said ill! . 6 tubular members being substantially equal tosaid focal length.

2'. In a telemeter of the self-contained base type, a pair of mainoptical systems comprising a pair of objectives, optical tube meansmounting said objectives on parallel axes in laterally spaced relationto each other a limiting distance measured along said axes less than thesum of the focal lengths of said objectives, and focusing means forvarying the relative distance along said axes between said objectiveswithin said first named distance.

" 3. In a telemeter of the self-contained base type, a pair of mainoptical systems comprising a pair of objectives, optical tube meansmounting said objectives on parallel axes in spaced relation to eachother a distance less than the sum of the focal lengths of saidobjectives, a reticle mounted in said optical tube means adjacent eachobjective in the image plane of the opposed objective, and focusingmeans for varying the relative distance between said opposed objectivesand reticles.

4. In a telemeter of the self-contained base type, a pair of mainoptical systems comprising a pair of objectives, a pair of optical tubesmeans mounting said objectives on parallel axes in spaced relation toeach other a distance less than the sum of the focal lengths of saidobjectives, a reticle mounted in each of said optical tube meansadjacent each objective in the image plane of the opposed objective.said optical tube means each comprising a plurality of sectionstelescopically joined intermediate said opposed objectives and reticles,and focusing means for relatively sliding said telescopically joinedsections.

5. In a telemeter of the self-contained base type, a pair of mainoptical systems comprising a pair of objectives of identical focallength, optical tube means mounting said objectives on parallel axes inspaced relation to each other a distance equal to said focal length, anda reticle mounted in said optical tube means adjacent each objective inthe image plane of the opposed objective; whereby said optical tubemeans may be reduced in length to substantially said focal length, andmeans having a coefiicient of expansion less than the coefiicient ofexpansion of steel, mounting each of said objectives and reticles insaid optical tube means.

6. In a telemeter of the self-contained base type, a pair of mainoptical systems comprising a pair of objectives of identical focallength, optical tube means mounting said objectives on parallel axes inspaced relation to each other a distance equal to said focal length, anda reticle mounted in said optical tube means adjacent each objective inthe image plane of the opposed objective; whereby said optical tubemeans may be reduced in length to substantially said focal length, andmeans comprising fused silica means mounting said objectives andreticles in said optical tube means.

'7. In a telemeter of the self-contained base type, a pair of mainoptical systems, each said system comprising an objective and a reticlealigned along respective axes, optical tube means mounting said axes inparallel spaced. relation to each other a distance less than the sum ofthe focal lengths of said objectives, said objectives being mounted atopposite ends of said optical tube means and each reticle being mountedin said tube means substantially coplanar with the objective of theother said system, and fused silica means for securing said tubularmembers to each other in said parallel relation.

8, 1n a telemeter of the selfecontained base type, a pair of mainoptical systems, each said system comprising an objective and a reticlealigned along respective axes, optical tube means mounting said axes inparallel spaced relation to each other a distance less than the sum ofthe local lengths of said objectives, said objectives being mounted atopposite ends of said optical tube means and each reticle being mountedin said tube means substantially coplanar with the objective of theother said system, and fused silica means for mounting said objectivesand reticles.

9. In a telemeter of the self-contained base type, a pair of mainoptical systems, each said system comprising an objective and a reticlealigned along respective axes, optical tube means mounting said axes inparallel spaced relation to each other a distance less than the sum ofthe ,focal lengths of said objectives, said objectives being mounted atopposite ends of said optical tube means and each reticle being mountedin said tube means substantially coplanar with the REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 943,109 Eppenstein Dec. 14, 19091,006,699 Straubel Oct. 24, 1911 1,313,495 Michelson Aug. 19, 19191,452,675 Carssow Apr. 24, 1923 1,869,417 Grunberg Aug. 2, 1932 FOREIGNPATENTS Number Country Date 139,437 Great Britain Mar. 4, 1920 416,398Great Britain Sept. 13, 1934

